As a method of forming a single α phase of a high crystalline gallium oxide thin film on a sample to be deposited, mist CVD is known (PLT 1). In this method, a gallium compound, such as gallium acetylacetonate, is dissolved in acid, such as hydrochloric acid, to prepare a raw-material solution, the raw-material solution is atomized to generate a raw-material mist, the raw-material mist is supplied to a film formation surface of a sample to form a film thereon with a carrier gas, and the raw-material mist is reacted at approximately 400° C. to form a thin film on the film formation surface, thereby forming a high crystalline gallium oxide thin film on the sample.